Ultraviolet radiation has been used for sterilization and killing microorganisms. For example, ultraviolet energy in the germicidal band for grape vines on a vineyard trellis has been used to provide a lethal exposure (irradiation) for the purpose of eradicating microorganisms, and stimulating host plant defense mechanisms. It is also known that crop yields can be enhanced by visible light exposure during night.
Recently, interest has increased in protected cultivation (e.g., greenhouses) to grow high quality local vegetables, especially early and late in the season. However, many coverings commonly used for greenhouses also block ultraviolet (UV) light. Most plants respond to UV light by initiating or increasing the synthesis of polyphenolic compounds, such as flavonoids, which absorb incident UV light and protect plant tissues from DNA damage that might otherwise result from UV light exposure. These compounds also are potent antioxidants and can affect cell metabolism (e.g., nitric oxide levels). Thus, the compounds may be important components of mammalian diets. Consequently, growing plants in protected cultivation may reduce their nutritional value.
A current approach to manipulate the UV environment in greenhouses utilizes coverings that allow UV light to pass through. However, such coverings have little benefit early and late in a growing season when ambient UV radiation is reduced because the sun is lower in the sky. Fluorescent sun lamps have been used to enhance polyphenolics in plants, but they are inefficient and, in addition, are breakable and contain mercury. Currently available deep UV light emitting diodes (DUV-LEDs) offer the possibility of a higher efficiency and safer UV light for protected cultivation, but these sources are still early in development and very dim.